Fair Value Measurements (Schedule Of Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, (Fair Value, Total)	$ 589,418
Liabilities, (Fair Value, Total)	142,128
Recurring Measurements, (Fair Value, Total)	447,290
2.875% Convertible Senior Notes Due 2038 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior notes, interest rate, percentage	2.875%
Trading Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, (Fair Value, Total)	419
Commodity Derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, (Fair Value, Total)	544,281
Liabilities, (Fair Value, Total)	130,094
Deferred Compensation Plan Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, (Fair Value, Total)	44,718
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, (Fair Value, Total)	44,969
Recurring Measurements, (Fair Value, Total)	44,969
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Trading Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, (Fair Value, Total)	251
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Deferred Compensation Plan Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, (Fair Value, Total)	44,718
Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, (Fair Value, Total)	544,449
Liabilities, (Fair Value, Total)	142,128
Recurring Measurements, (Fair Value, Total)	402,321
Significant Other Observable Inputs (Level 2) [Member] | Trading Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, (Fair Value, Total)	168
Significant Other Observable Inputs (Level 2) [Member] | Commodity Derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, (Fair Value, Total)	544,281
Liabilities, (Fair Value, Total)	130,094
Interest Rate Derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities, (Fair Value, Total)	12,034
Interest Rate Derivatives [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities, (Fair Value, Total)	$ 12,034